Solvent extraction process



2,865,852 SOLVENT EXTRACTION PROCESS James A. Davies, Scarsdale, N. Y.,assignor to The Texas Company, New York, N. Y., a corporation ofDelaware Application May 21, 1954, Serial No. 431,363

2 Claims. (Cl. 208-327) This invention relates to the solvent extractionof liquid mixtures wherein a liquid mixture is separated into itscomponents by solvent extraction. More particularly, this inventionrelates to the liquid-liquid extraction of mixtures by solventextraction wherein two separate liquid phases are flowedcountercurrently with respect to each other in an extraction zone.

In countercurrent liquid-liquid extraction wherein two phases areproduced, a so-called raflinate phase and a socalled extract phase,these separate phases being produced at opposite ends of an extractionzone, it has been observed in some instances that one of the phases,such as the raffinate phase, tends to become entrained and producedalong with the other phase, such as the extract phase. It is believedthat when this condition obtains the density or specific gravitydilference between the two separate phases within the extraction zone isinsufficient to permit a reasonably and su'fiiciently rapid andefiicient separation of the phases, one from the other, with the resultthat one phase tends to become entrained and pro duced along with theother phase. By way of explanation and assuming the simple case whereinthe liquid mixture, such as a petroleum fraction, is contacted with asingle selective solvent so as to produce a raflinate phase and anextract phase, although the specific gravity diiference between thepetroleum fraction being treated and the selective solvent employedoriginally may be rather high, e. g., about 0.3 and greater, theselective solvent employed having a higher specific gravity than thepetroleum fraction being treated, as the selective solvent flowscountercurrently in contact with the petroleum fraction some of thecomponents of the petroleum fraction are taken up into the selectivesolvent to form an extract phase having a specific gravity substantiallyless than the specific gravity of the selective solvent itself. Also,some of the selective solvent becomes dissolved in the petroleumfraction being treated to form what may be called the raflinate phasewith the result that the raflinate phase has a specific gravitysubstantially greater than the petroleum fraction being treated. hisapparent therefore that the specific gravity difference between theextract and rafiinate phases will be considerably smaller than thedifference between the specific gravity of the original selectivesolvent and the original petroleum fraction.

It has been observed during the furfural refining of a petroleumfraction, such as a heavy cycle gas oil, a considerable amount of theoil or rafiinate phase is entrained and. produced along with the extractphase leaving the bottom of the extraction tower. This condition morefrequently occurs when the petroleum fraction is treated with arelatively small solvent dosage, that is, about one volume of furfuralper volume of petroleum fraction being treated. To prevent and eliminatethis entrainment or carryunder of the oil phase along with the extractphase, the solvent dosage of furfural has been substantially increasedin order to increase the specific gravity difference between theraffinate or oil phase and the extract or furfural phase. Unfortunatelywhen higher solvent dosages atetf 2,865,852 Patented Dec. 23, 1958 ofselective solvent are employed the particular petroleum fraction beingtreated is over-refined, resulting in decrease yields of raflinate phaseor refined oil. Additionally, substantially larger quantities of extractphase (solvent) must be treated, handled and recovered.

It is an object of this invention to provide an improved liquid-liquidextraction process. f

It is another object of this invention to provide an improvedliquid-liquid extraction process where two separate liquid phases areflowed countercurrently with respect to each other.

It is still another object of this invention to provide a liquid-liquidcountercurrent extraction process wherein entrainment of one liquidphase, such as a raffinate phase, in another liquid phase, such as theextract phase, is substantially eliminated in the produced ratlinate andextract phases. y

In atleast one embodiment of this invention at least one of theforegoing objects will be obtained. How these and other objects of thisinvention are accomplished will are present in the extraction zone andwherein the liquid mixture to be separated is introduced into theextraction zone intermediate the ends thereof or at said one end andwherein a partially miscible llquid selective solvent is introduced atsaid other end of the extraction zone, im-.

proved results are obtained by introducing additional selective solventinto said extraction zone at a point downstream from said other endof'the extraction zone in the direction of flow of said selectivesolvent through said extraction zone. Accordingly, in a liquid-liquidcountercurrent extraction operation wherein two separate liquid phasesare in intimate contact with each other and wherein said separate liquidphases tend to approach substantially the same specific gravity with theresult that a portion of one of the liquid phases, such as the raflinatephase, tends to become entrained in and produced along with the otherliquid phase, such as the extract phase, the separation of the separateliquid phases within the extraction zone is enhanced and improved byinjecting an additional amount of selective solvent into the extractionzone so as to increase the density or specific gravity differentialbetween the produced rafiinate and extract phases within the extractionzone. In the practice of this invention it is desirable that sufiicientadditional selective solvent be injected into the extraction zone sothat the minimum specific gravity differential between the separateliquid phases within the extraction zone is maintained at a value notless than 0.1, preferably at least 0.2.

Referring now to the drawing which schematically illustrates thepractice of my invention as applied to a vertical or upright extractionzone, the extraction zone is indicated at 11. The extraction zone 11 maybe considered as made up of a contacting or refining zone 11asuperimposed upon a settling zone 11b. The practice of this invention isparticularly applicable to an isothermal operation of extraction Zone11, i. e., wherein substantially the same temperature prevailsthroughout the extraction zone. It is realized, however, that atemperature gradient may also be employed in the operation of extractionzone 11. The liquid mixture to be fractionated into its respectivecomponents is introduced into the extraction zone 11 via conduit 12 anddistributor 12a. In the embodiment illustrated in the drawing, aselective solvent which is more dense than and partially miscible withthe feed mixture 'is introduced into the upper end of the extractionzone via'conduit Maud-distributor 14a. In operation, the less dense feedmaterial tends to rise within the extraction zone countercurrently withrespect to the downwardlyflowing moredense selective solvent. Contactingmasses with the extraction zone 11 are schematically illustrated at 15a,15b, 15c and 15d, respectively. These contact masses serve to promoteand facilitate liquid-liquid mixing and contact between the downwardlyflowing solvent and the upwardly flowing feed mixture. These contactmasses may comprise a porous mass of granular material, or a mass of-Berl saddles, Raschig rings or the'like. Thesecontact masses may bereplaced by a rotating disc co-ntactor.

Two separate liquid phases are present within the extraction zone, aliquid phase comprising predominantly selective solvent together with aportion of the components selectively extracted from the feed mixtureundergoing fractionation and other liquid phase comprising predominantlysubstantially the original feed mixture (certain of the componentsoriginally present having been removed) together with a small amount ofselective solvent dissolved therein. These separate-phases areseparately recovered at opposite ends of the extraction zone 11. Araffinate phase is withdrawn from'the upper end of the extraction zonevia conduit 16. This raffinate phase will comprise certain of thecomponents originally present in the feed mixture together with anamount of selective solvent dissolved therein. An extract phase iswithdrawn from the lower end of the extraction zone via conduit 18. Thisextract phase will comprise mainly selective solvent together withcertain components originally present in the feed mixture.

Separation of the separate liquid phases within the extraction zone isimproved and enhanced by introducing thereinto selective solvent at apoint downstream from the pointof injection of selective solvent viaconduit 14. The additional selective solvent is injected into extractionzone 11 at the most suitable point to'increase the specific gravitydifferential between the two separate liquid phases contained therein.Accordingly, this additional selective solvent may be introduced intothe extraction zone 11 at about the middle or lower portion of thecontacting zone (refining section) of the extraction Zone, above thepoint of introduction of the feed mixture to be separated, for example,the additional selective solvent may be introduced into extraction zone11 via valve conduit and distributor 20a". Preferably this additionalselective solvent is introduced into the settling section of theextraction zone below the point of introduction of the feed mixture viavalve conduit 21 and distributor 21a. Under normal and satisfactoryoperating conditions obtained in the practice of my invention, aliquid-liquid interface 22 is established in the settling section of theextraction zone.

Exemplary of the advantages to be obtained in the practice of myinvention as applied to the furfural solvent refining of a heavy cyclegas oil, a considerable amount of carryunder or entrainment of the oilphase along with the extract phase leaving the bottom. of the extractiontower was experienced during conventional operation wherein all thefurfural was introduced into the top of the tower. The solvent dosage offurfural to the oil being treated was increased in order to improvephase separation and to avoid carryunder. By increasing the solventdosage, however, the gas oil was refined with the .result that the yieldof refined oil was substantially reduced. However in accordance withthis invention in order to prevent carryunder or entrainment of theraffinate phase, additional selective solvent was introduced into theextraction zone at the lower portion thereof, for example, into thesettling section of the extraction zone. At the same time it was foundthat the overall solvent feed ratio was reduced by about half and stillproduced a satisfactorily refined oil. There was recovered as rat finatea refined gas-oil fraction in substantially increased yield. The resultsare set forth in Table l herewith.

1 Solvent dosage may be defined as the volume ratio of solvent to charge011 and in the practice of this invention the total solvent dosage mayhave a val e in the range 25200%, more or less, depending upon theproperties of the charge oil.

The practice of this invention is broadly applicable to anyliquid-liquid solvent extraction process wherein two separate liquidphases are flowed countercurrently with respect-to each other.Accordingly, this invention is suitable not only for those liquid-liquidcountercurrent solvent extraction operations employing one selectivesolvent in contact with a liquid mixture to be fractionated but is alsoapplicable to the so-called Duo Sol solvent extraction process whereinthe liquid mixture to be separated is introduced at some intermediatepoint of the extraction zone wherein two immiscible solvents ofdifferent densities are flowing countercurrently. The practice of thisinvention is particularly applicable to the petroleum industry for thesolvent refining of various petroleum fractions, such as gasolines,naphthas, kerosenes, gas oils, e. g., catalytic cycle gas oils andsimilar boiling virgin distillates, lubricating oils, and the like.Selective solvents which may be used in the practice of this inventionmay be more dense or less dense than the feed mixture to be separatedand may comprise such well-known selective solvents as nitrobenzene,liquid propane and sulfur dioxide, fl,,B-dichloroethyl ether (Chlorex),phenols (Selecto), furfural, liquid hydrogen fluoride and the variousother well-known organic and inorganic selective solvents.

In the practice of this invention, the ordinary temperatures andpressures usually employed in conventional solvent extraction processesare employed. It 1s mentioned that the additional selective solventintroduced into the extraction zone to enhance and improve the phaseseparation of the separate liquid phases therein may be supplementedand/or replaced entirely by the extract phase recycled directly into theextraction zone in the manner in accordance with this invention andwithout any intervening treatment or change in composition.

Many variations, modifications and substitutions are possible in thepractice of this invention without departing from the spirit and scopethereof.

I claim:

1. In the liquid-liquid countercurrent solvent extraction of a catalyticcycle gas oil to be fractionated to produce at one end of an extractionzone an extract phase and at the other end of said extraction zone araffinate phase, said extraction zone comprising two superposedcontiguous sections, a refining section ad acent said other end of saidextraction zone and a settling section adjacent said one end of saidextraction zone and wherein furfural, a partially miscible selective solvent, is introduced into said extraction zone at said refining sectionand said catalytic cycle gas oil is introduced into said extraction zoneat about that position where said refining section and said settlingsection meet, the specific gravity differential between said catalyticcycle gas oil and said furfural selective solvent being substantial,said selective solvent having a higher specific gravity than saidcatalytic cycle gas oil, and wherein two contiguous continuous separateliquid phases, said raffinate phase and said extract phase, are presentwithin said settling section and tend to have substantially the samespecific gravity, the interface between said liquid phases being locatedin said extraction zone within said settling section, the improvementwhich comprises introducing additional furfural selective solvent intosaid extraction zone below the point of introduction of said catalyticcycle gas oil thereinto at said settling section directly into saidraffinate phase therein to maintain the specific gravity diflerentialbetween said separate liquid phases in said settling section greaterthan 0.1.

2. In the liquid-liquid countercurrent solvent extraction of a catalyticcycle gas oil to be fractionated to produce at the lower end of avertically extending extraction zone an extract phase and at the upperend of said extraction zone a rafiinate phase, said extraction zonecomprising two superposed contiguous sections, a refining sectionadjacent said upper end of said extraction zone and a settling sectionadjacent said lower end of said extraction zone, wherein furfural, apartially miscible selective solvent, is introduced into said extractionzone at said refining section and said catalytic cycle gas oil isintroduced into said extraction zone at about the position therein wheresaid refining section and said settling section meet, the specificgravity differential between said catalytic cycle gas oil and saidfurfural selective solvent being substantial, said furfural selectivesolvent having a higher specific gravity than said catalytic cycle gasoil, and wherein two contiguous continuous separate liquid phases, saidraffinate phase and said extract phase, are present within the saidsettling section and tend to have substantially the same specificgravity, the interface between said liquid phases being located withinsaid settling section, the improvement which comprises introducingadditional furfural selective solvent into said extraction zone belowthe point of introduction of said catalytic cycle gas oil thereinto atsaid settling section directly into said rafiinate phase therein tomaintain the specific gravity diiferential between said separate liquidphases in said settling section greater than about 0.2.

References Cited in the file of this patent UNITED STATES PATENTS2,037,318 Fenske et al Apr. 14, 1936 2,054,295 Merrill Sept. 15, 19362,149,574 Brown Mar. 7, 1939 2,346,491 Kiersted Apr. 11, 1944 2,525,813Fragen Oct. 17, 1950 2,564,071 Lien et al Aug. 14, 1951 2,626,230 ClarkeJan. 20, 1953 2,692,222 Packie Oct. 19, 1954

1. IN THE LIQUID-LIQUID COUNTERCURRENT SOLVENT EXTRACTION OF A CATALYTICCYCLE GAS OIL TO BE FRACTIONATED TO PRODUCE AT ONE END OF AN EXTRACTIONZONE AN EXTRACT PHASE AND AT THE OTHER END OF SAID EXTRACTION ZONE ARAFFINATE PHASE, SAID EXTRACTION ZONE COMPRISING TWO SUPERPOSEDCONTIGUOUS SECTIONS, A REFINING SECTION ADJACENT SAID OTHER END OF SAIDEXTRACTION ZONE AND A SETTLING SECTION ADJACENT SAID ONE END OF SAIDEXTRACTION ZONE AND WHEREIN FURFURAL, A PARTIALLY MISCIBLE SELECTIVESOLVENT, IS INTRODUCED INTO SAID EXTRACTION ZONE AR SAID REFININGSECTION AND SAID CATALYTIC CYCLE GAS OIL IS INTRODUCED INTO SAIDEXTRACTION ZONE TO ABOUT THAT POSITION WHERE SAID REFINING SECTION ANDSAID SETTLING SECTION MEET, THE SPECIFIC GRAVITY DIFFERENTIAL BETWEENSAID CATALYTIC CYCLE GAS OIL AND SAID FURFURAL SELECTIVE SOLVENT BEINGSUBSTANTIAL, SAID SELECTIVE SOLVENT HAVING A HIGHER SPECIFIC GRAVITYTHAN SAID CATALYTIC CYCLE GAS OIL, AND WHEREIN TWO CONTIGUOUS CONTINUOUSSEPARATE LIQUID PHASES, SAID RAFFINATE PHASE AND SAID EXTRACT PHASE, AREPRESENT WITHIN SAID SETTLING SECTION AND TEND TO HAVE SUBSTANTIALLY THESAME SPECIFIC GRAVITY, THE INTERFACE BETWEEN SAID LIQUID PHASES BEINGLOCATED IN SAID EXTRACTION ZONE WITHIN SAID SETTLING SECTION, THEIMPROVEMENT WHICH COMPRISES INTRODUCING ADDITIONAL FURFURAL SELECTIVESOLVENT INTO SAID EXTRACTION ZONE BELOW THE POINT OF INTRODUCING OF SAIDCATALYTIC CYCLE GAS OIL THEREINTO AT SAID SETTLING SECTION DIRECTLY INTOSAID RAFFINATE PHASE THEREIN TO MAINTAIN THE SPECIFIC GRAVITYDIFFERENTIAL BETWEEN SAID SEPARATE LIQUID PHASES IN SAID SETTLINGSECTION GREATER THAN 0.1.